The field of art to which this invention is directed is hydrogenation catalysts.
Fatty alcohols, i.e., aliphatic, predominantly linear primary alcohols with chain lengths greater than eight carbon atoms represent especially significant intermediate products in the chemical industry. One of their primary uses is in the production of surfactants, such as fatty alkyl sulfates, polyglycol ethers or polyglycol ether sulfates.
The most important raw materials for the production of these alcohols are fatty acids, or fatty acid esters, in the form of mixtures of various chain lengths, which can be obtained for instance, from natural fats and oils. Conversion into fatty alcohols occurs through catalytic hydrogenation under pressure, for which catalysts based on copper-chromium have proven to be particularly effective.
The hydrogenation reaction is conducted as suspension hydrogenation, as vapor-phase hydrogenation, or in the trickle phase. Sufficiently high reaction rates are only achieved at pressures above 250 bar and temperatures in the 260.degree. to 300.degree. C. range. As a rule, the triglycerides are transesterified with methanol according to known methods before hydrogenation, the free fatty acids being esterified. Nevertheless, the reaction mixture contains a residual concentration of free carboxylic acids.
An important technical embodiment is represented by the suspension hydrogenation of fatty acids according to the LURGI process. In this process, the fatty acid mixture is continuously fed to the hydrogenation reactor and esterified in situ by the excess fatty alcohol present.
The presence of the free fatty acids obviously makes great demands on the acid resistance of the catalysts that are used. The catalyst metals--copper, in particular--can be eluted due to the attack of the acids, so that the catalyst's efficacy is impaired. Moreover, this also leads to contamination of the product.
A possible means of reducing the acid solubility of copper-chromium catalysts, is extraction with acetic acid. However, not only is this process particularly expensive, but the hydrogenation activity of the resulting catalyst is significantly reduced, which is not surprising, given the loss of active copper metal through the treatment with acetic acid.
It is further known that the crystallinity of copper-chromium catalysts can be increased through calcination at comparatively high temperatures, whereas the phases present are completely or partially converted into CuCr.sub.2 O.sub.4 that has spinel structure depending on the copper-chromium molar ratio. As a result of this heat treatment, on the one hand the solubility in acid of the catalyst metals decreases, but on the other hand the inner surface of the catalyst, as determined by the BET method, is reduced. At the same time a drastic reduction in hydrogenation activity is observed.
Copper oxide-chromium oxide catalysts have also been used in other areas of application. For instance, German Patent No. DE-B-23 19 551 describes catalysts for the detoxification of exhaust gases, in particular from automobile engines, that contain aluminum, chromium, and copper oxide as substantial components. These catalysts are produced through mixing of an aluminum oxyhalide sol and a chromium oxyhalide sol, calcination of the mixture and impregnation of the calcinated product with a compound that supplies copper oxide. The catalyst need not be acid-resistant for the purpose indicated.
Copper chromite catalysts for the oxidation of the oxidizable hydrocarbons and carbon monoxide present in the exhaust gases of internal combustion engines are further known from U.S. Pat. No. 3,855,388 (German Patent Application No. DE-A-22 00 460). These catalysts are produced by mixing an aluminum oxide supporting material, of which at least 20 percent is present in hydrogenated condition, with copper chromite and then drying the product obtained in order to remove the water. These catalysts also need not be acid-resistant for the purpose indicated.
U.S. Pat. No. 3,870,658 similarly relates to copper chromite-aluminum oxide catalysts for the oxidation of carbon monoxide from automobile exhaust gases. The catalysts are obtained through the mixing of aqueous solutions of aluminum salts or aluminum oxide, copper salts, and chromium salts or chromium oxide, drying of the solutions and sintering of the product. Thereafter, the excess copper oxide is removed through treatment of the product with a strong mineral acid. As a result of this acid treatment, the catalysts obtained are probably acid-resistant; however, they do not have sufficient hydrogenation activity, since they do not contain promoting agents.
Copper chromite-aluminum oxide catalysts that can be used for hydrogenation reactions are further known from U.S. Pat. No. 3,756,964 (German Patent No. DE-B-23 11 114). A porous supporting material, such as aluminum oxide or silicic acid, is used to produce these catalysts, with copper oxide and chromium oxide precipitated in the pores of this supporting material. These oxides are converted into copper chromite through heating. Through this type of production, it is guaranteed that no copper or chromium compounds will enter the wash water when the catalyst is washed out. The catalysts, however, have insufficient hydrogenation activity.
Copper chromite catalysts that can be used as hydrogenation catalysts are further described in German Patent Application No. DE-A-22 46 382. These catalysts are produced by dissolving metallic copper in the presence of oxygen in an ammonium carbonate solution and mixing the tetraamine copper complex obtained with chromic acid or a solution of copper dichromate; thereafter, the basic copper ammonium chromate produced is calcinated in order to form a copper chromite catalyst. The copper chromite can also be precipitated on an inert supporting material, such as aluminum oxide. The activity of the catalysts obtained is, however, relatively small since the catalysts contain no promoters.
Copper chromite catalysts for the oxidation of hydrocarbons and carbon monoxide from the exhausts of combustion engines are also known from U.S. Pat. No. 3,789,022. In order to produce these catalysts, copper, cerium and chromium oxide layers are precipitated one after another on an aluminum oxide support. However, these catalysts have a low acid resistance.
Finally, a process in which catalysts based on copper oxide, which may contain chromium oxide, aluminum oxide and the oxides of lanthanoids among others, are used for the hydrogenation of carboxylic acid esters into alcohols is known from European Patent Application-A-0 210 795.
The objective of the present invention is to produce catalysts that combine significantly improved acid resistance with high activity in the hydrogenation of fatty acids or fatty acid mixtures and/or the esters thereof.